The present invention relates to a photoelectric transducer and a photoelectric transducer element array.
At present, signals that are propagated between semiconductor chips such as LSI (Large Scale Integration) chips or the like are in the form of electric signals transmitted through substrate interconnections. As MPUs available today are becoming higher and higher in functionality, the amount of data exchanged between semiconductor chips grows to such an extent that various high-frequency problems have arisen with respect to the data. Typical examples of those problems include RC signal delay, impedance mismatching, EMC/EMI, crosstalk, etc.
Attempts have been made in the electronic packaging technology and related fields of art to solve the above problems by way of optimization of interconnection layouts, development of new materials, etc.
However, the attempts to optimize interconnection layouts and develop new materials are being hampered by limitations due to material properties. For achieving higher system functionality, it is necessary to reconsider the structure itself of printed-wiring boards that are designed for mounting simple semiconductor chips thereon. There have been proposed in recent years various fundamental solutions to the above problems. Typical proposals will be described below.                Fine Interconnections Based on Multichip Modules (MCM):        
A high-functionality chip is mounted on a precision mounting board of ceramics and silicon, and fine interconnections which could not be formed on mother boards (multilayer printed boards) are realized on the precision mounting board. The interconnections thus formed have a reduced pitch, allowing buses to have a larger width for transmitting a much larger amount of data.                Electric Interconnections on Various Sealed and Integrated Semiconductor Chips:        
Various semiconductor chips are two-dimensionally sealed and integrated by polyimide resin, and fine interconnections are formed on the integrated board. The interconnections thus formed have a reduced pitch, allowing buses to have a larger width for transmitting a much larger amount of data.                Three-Dimensional Coupling of Semiconductor Chips:        
Through electrodes are formed in various semiconductor chips, and the semiconductor chips are stacked into a multilayer structure. The semiconductor chips are interconnected by the through electrodes which provide physically short interconnections. The physically short interconnections are effective to avoid problems such as signal delays. However, the multilayer structure tends to produce a large amount of heat and develop thermal stresses between the semiconductor chips.
To send and receive signals at higher speeds for the transmission of larger amounts of data, there has been developed an optical transmission coupling technology employing optical interconnections. For details, see Nikkei Electronics, “Encounter with optical interconnections”, Dec. 3, 2001, pages 122, 123, 124, 125, FIGS. 4, 5, 6, 7, and NTT R&D, vol. 48, no. 3, pages 271-280 (1999), for example. Optical interconnections are applicable to various locations such as between electronic devices, between boards in electronic devices, and between chips in boards.
For example, as shown in FIG. 13A of the accompanying drawings, a conventional photoelectric transducer 50 has an optical waveguide 52 mounted on a printed-wiring board 51, a plurality of light-emitting devices 53 each including a surface emitting laser, for example, and a plurality of light-detecting devices 55 each including a photodiode. Light 54, e.g., a laser beam, that is modulated with a signal by each of the light-emitting devices 53 is applied to the optical waveguide 52. The light 54 is guided through the optical waveguide 52, and emitted from the optical waveguide 52 and detected by each of the light-detecting devices 55. The photoelectric transducer 50 provides an optical transmission-communication system which employs the optical waveguide 52 as a transmission path for a signal-modulated laser beam or the like.
The optical waveguide 52 includes a pair of cladding layers 56a, 56b and a core layer 57 sandwiched between the cladding layers 56a, 56b. As shown in FIG. 13B of the accompanying drawings, the optical waveguide 52 actually has a plurality of parallel core layers 57 sandwiched between the cladding layers 56a, 56b. 
As shown in FIG. 13C of the accompanying drawings, the light-emitting devices 53 and the light-detecting devices 55 are mounted respectively on substrates 58a, 58b in alignment with entrance ends 59 and exit ends 60 of the core layers 57, providing a light-emitting device array 61 and a light-detecting device array 62. Anode electrodes 63 are electrically connected respectively to the light-emitting devices 53, and cathode electrodes 64 are associated with respective pairs of the light-emitting devices 53 and the anode electrodes 63. Similarly, anode electrodes 63 are electrically connected respectively to the light-detecting devices 55, and cathode electrodes 64 are associated with respective pairs of the light-detecting devices 55 and the anode electrodes 63.
The anode electrodes 63 and the cathode electrodes 64 of the light-emitting device array 61 and the anode electrodes 63 and the cathode electrodes 64 of the light-detecting device array 62 are electrically connected through solder bumps 65 to a drive circuit 67 mounted on an interposer board 66.
In the conventional photoelectric transducer 50 shown in FIGS. 13A through 13C, the cathode electrodes 64 are associated with respective the pairs of the light-emitting devices 53 and the anode electrodes 63, and the cathode electrodes 64 are associated with respective the pairs of the light-detecting devices 55 and the anode electrodes 63. The cathode electrodes 64 are electrically connected to respective terminals (not shown) of the interposer board 66 by the solder bumps 65. It is necessary that adjacent two of the cathode electrodes 64 be spaced apart from each other to keep adjacent two of the solder bumps 65 from overlapping each other when they are connected. Therefore, the boards 58a, 58b need to have a wide area, making it difficult to reduce the sizes of the light-emitting device array 61 and the light-detecting device array 62, and resulting in an increase in the cost thereof.